The complex nature of the emulsions of water in crude oil is one of the main drawbacks to the development of techniques suitable for demulsification and phase separation in the oil industry. In spite of the huge recent efforts for developing dependable and efficient demulsification techniques, most emulsions of water in crude oil cannot be broken in reduced times. Actually the demulsification operation is a key process for removing water from crude oil in production platforms and refineries. Specifically, in order to remove water soluble salts from crude oil up to acceptable levels, there is a need of demulsification (dehydration or desalting) stages in the desalting plants generally encountered in refineries.
Interfacial films on the surface of the dispersed water droplets hinder droplet coalescence, causing stabilization of water emulsions in crude oil.
Surface active species found in crude oil such as asphaltenes, resins, oil-soluble organic acids, solids and paraffin compounds are among the materials constituting said interfacial films. Since some of these compounds contain ionizable groups, it is expected that the pH of the aqueous phase can affect ionization of these groups in the interfacial films, yielding radical changes in the physical properties of films as well as in the solubility of some polar organic compounds relative to the aqueous phase. Thus, a certain number of demulsification techniques for separating water and oil have been applied in the oil industry, including chemical demulsification and pH adjustment. Further, gravity or centrifugation deposition techniques, filtration, thermal treatment, membrane separation and electrostatic demulsification are also applied in the oil industry.
However, the demulsification of water-in-oil emulsions of highly viscous crude oils can be very laborious, leading to excessively time-consuming procedures. Besides, there is the hard job of relating the properties of a crude oil emulsion (which can usually be evaluated in the laboratory, such as water content, salinity, pH, asphaltene content, interfacial film properties and others) and the separation conditions (such as temperature, kind and amount of surface agent, residence time, intensity of electric field, among others), the quantification of it being usually assessed after time-consuming tests and calibrations.
Microwave irradiation is being studied as a tool for demulsification. This is due to the fact that microwave irradiation offers a clean, cheap and convenient heating process that in most of times results into better yields and shorter reaction times. It is considered that reaction acceleration by microwave exposition and also the phenomena involved in demulsification result from wave interactions with the material, leading to thermal effects estimated by temperature measurements (dielectric heating) and specific effects (not purely thermal) generally connected to the selective absorption of microwave energy by polar molecules.
The heating of liquids using microwaves can be explained by the interaction of matter with the electric field of the incident radiation, causing the movement of ions as well as that of induced or permanent molecule dipoles. The movement of such species can cause heat generation. The two main dielectric heating mechanisms are: dipole rotation and ionic conduction, both being reported below in the present specification.
Electric dipoles are formed by the redistribution of electric charges. The action of an electric field causes the orientation of dipole moments parallel to the electric field, while the action of an electromagnetic field results in the rotation of the dipoles caused by the high number of times that the electromagnetic field is alternated.
In liquids, the electric dipoles cannot rotate instantaneously and the time required for the movement of the dipoles depend on the molecular mass, on the viscosity of the medium and on the forces exerted by the neighboring molecules. For low radiation frequencies, the time where the electric field changes direction is higher than the dipole response time. Thus, the electric field is in phase with polarization. The energy supplied by the electric field is employed in the rotation and there is nearly no transformation of electromagnetic energy into heat. For very high radiation frequencies, dipoles cannot follow the electric field changes and the molecules do not move. For frequencies comprised between those two limiting cases, electric dipoles slightly delay with respect to electric field variations and a portion of the energy that the electric field provides for dipole rotation is stored. Such energy will be turned into heat resulting from the friction with neighboring molecules. This heating mechanism is called dipole rotation. When the irradiated sample is an electric conductor or semiconductor formed by ions (such as NaCl aqueous solutions), these ions can move through the material so as to follow the variations in electric field. The resulting electrical currents heat the sample as a consequence of the electrical resistance. This mechanism is called ionic conduction.
During the microwave heating of a material, radiation penetrates the material so that heat transfer occurs from within the material up to the surface of it. This kind of transfer causes the global warming of the material and a quick increase of its temperature.
This kind of heating is quite different from conventional heating which depends on the thermal conductivity of the material, on temperature gradients created throughout the material and on convection currents. Conventional heating is characterized by low rates in temperature increase.
Besides the quick heating of the materials, other advantages can be attributed to dielectric heating relative to conventional heating such as for example:                Selective heating;        Miniaturization of equipment;        Low electrical power consumption cost;        Reduced environmental pollution;        
Since the separation of water-in-oil and oil-in-water emulsions employing microwave has been only slightly explored, the literature on this application is scarce.
Broadly, high rates are observed for the separation of water and oil phases, the good results being backed by the mechanisms listed below.                Microwaves quickly heat emulsions, reducing the viscosity of the continuous phase (water-in-oil emulsions), thus favoring the contact among water drops;        The temperature increase can cause reduction in the viscosity of the rigid film formed by natural surface agents in the water-oil interface, making easier the coalescence between said dispersed drops;        The microwave-induced molecular rotation neutralizes the zeta potential of the dispersed drops, thus reducing the stabilization offered by the ionic surface agents;        The preferred absorption of microwaves by water drops in water-in-oil emulsions causes intense internal pressure in these drops, resulting in the expansion of the dispersed phase and in the reduction of the thickness of the interfacial film.        
Among these mechanisms, viscosity reduction resulting from temperature rise and reduction of zeta potential when water-in-oil emulsions are exposed to microwaves have been experimentally validated. Those mechanisms based on the increase of pressure within water drops are not proved. On the other hand, the breaking of chemical bonds cannot be induced by the mere adsorption of microwaves in view of the low energy of photons as compared to the chemical bonds-associated energies. The energy range of hydrogen bonds, ionic bonds and Van der Waals-like interactions vary between 0.04 and 4.51 eV, while the microwave photons are associated to energies varying between 1.24×10−6 and 1.24×10−4 eV according to the microwave frequency.
The scientific literature as well as patented documents on the separation of water-in-oil emulsions with microwaves will be discussed below in the present specification.
Chang, C. C., Chen, C., Demulsification of W/O emulsions by microwave radiation, Sep. Sci. Technol., 37(15), 3407-3420, 2002 studied the effect of salts and inorganic acids solubilized in the aqueous phase on the efficiency of the separation of water-in-oil emulsions under the action of microwaves. This author has reached the conclusion that the addition of low concentrations of electrolytes and acids raises the demulsification rate of the mixtures. The addition of high electrolyte concentrations can limit the dipole rotation of the water molecules and reduce the efficiency of the phase separation. For the NaCl electrolyte, increased demulsification rates were observed for lower-than 0.5M concentrations. The booster effect on the demulsification for low electrolyte concentrations is related to the increase in the loss factor of these solutions. The influence of the water drops size was also studied by the same author, who found that the demulsification rate rises with the dispersed phase drop size. The experiments were carried out in a conventional microwave oven.
Xia L., Lu, S., and Cao, G., Stability and demulsification of emulsions stabilized by asphaltenes or resins, J. Colloid Interf Sci., 271, 504-506, 2004 have investigated the effect of asphaltene and resin concentration as well as their colloidal state on the demulsification rate of water-in-oil emulsions. In this work it could be observed that for low concentrations of asphaltenes and resins, these species contribute to the stability of the emulsions. On the other hand, the relatively high asphaltene and resin concentrations can favor the building of aggregates that are slightly soluble in the oil phase, those being devoid of stabilizing properties. The demulsification rates of emulsions employing microwaves and conventional heating were also compared. Authors found that the utilization of microwaves increase in one order of magnitude the coalescence rate of the dispersed drops. The experiments were carried out in a conventional microwave oven.
U.S. Pat. No. 4,582,629 describes the first approach directed to the use of microwaves for separating water-in-oil and oil-in-water emulsions. The relevance of this study stems from the fact that a new technique which can replace conventional thermal treaters is advanced. The author also suggests the use of microwaves as a complementary process to thermal heating. Based on the technology exposed in this patent, improvements were developed aiming at increasing the efficiency of emulsion separations. To this end, modifications in the process or in equipment design were proposed.
Accordingly, U.S. Pat. No. 4,810,375 teaches a process for the separation of oil-in-water emulsions using microwaves where the separated water is continuously re-circulated in order to increase the process efficiency. It is further described that the proposed process can be coupled to conventional separation systems, either upstream or downstream the microwave system. This patent refers to the separation of oil-in-water emulsions using the system of the U.S. Pat. No. 4,582,629 with water recirculation.
U.S. Pat. No. 4,853,119 describes the use of a coalescer medium (26) in cavity (14) of U.S. Pat. No. 4,582,629, said coalescer being of any suitable high surface area configuration, such as wood mesh excelsior or corrugated polypropylene. Other high surface area materials may be used, provided they have a low dielectric constant and a low loss factor. Preferred materials are those having a dielectric constant at 2450 MHz of from about 0.1 to about 15, most preferably from 2 to 3, and a loss factor of from near zero to about 2, most preferably less than 0.05.
U.S. Pat. Nos. 6,077,400 and 6,086,830 describe a unit formed by two juxtaposed cavities constructed from conductive materials separated by a central waveguide. It is alleged that in such cavities the microwaves are quickly reflected and resonance patterns lead to high microwave absorption rates. The upward flow of emulsion throughout both cavities aims at delaying the deposition of solids such as sands. The entry of the emulsion in this unit is preceded by a heating step using conventional techniques which take the fluid temperature to the 49° C. to 52° C. ranges. It is considered that this heating step makes possible, on the one hand, the melting of some solids such as waxes and dirt so as to increase the emulsion dielectric constant, and on the other hand, the viscosity reduction of the emulsions, which makes their flow easier. In this unit, the temperature in the cavities has been kept constant with the aid of modifications in the inlet flow rate of the emulsion in the cavities. This patent document describes a two-step process for emulsion separation comprising exposing the emulsion to microwaves using the unit of U.S. Pat. No. 4,582,629 followed by complete phase separation by centrifugation or gravity separation in a separator.
In the international publication WO 01/12289 is described a microwave process for water-in-oil emulsion separation based on the choice of optimum radiation frequencies for which the power consumed in the process is minimized. In this process the dwell time of samples exposed to radiation is lower than 5 seconds, during which the emulsion temperature increases 10° C. Optimum frequency should be chosen as a function of the experimental conditions such as temperature, salt concentration, water drop size and water percentage in the emulsion. Applicants state that such frequency should secure the selective water heating, avoiding microwave-oil interaction while at the same time maximizing the dielectric loss factor. This factor is related to the efficiency of the matter (in this case water) to convert electromagnetic energy into heat. The loss factor has maximum values depending on the kind of fluid to be heated. Thus for oils the maximum values found for the loss factor is at frequencies lower than 1 GHz, for water such frequencies are found near to 20 GHz and for brine frequencies lower than 3 GHz result into optimum loss factors. On the other hand, the optimum radiation frequency is influenced by the water drop size. The drop diameter should be quite lower than the radiation penetration depth. It should be borne in mind that the penetration depth is reduced with the frequency increase, thus, for 20 GHz frequencies penetration depths of 3 mm can be identified. Applicants point out the relevance of including an agitation system to favor the contact among water drops. Additional information related to the process such as the kind of oil or the temperature and working power are not included in the published document. There is also no mention as to on-line monitoring or to adjustment of operation conditions resulting from the obtained results.
In spite of existing processes, the current available technology does not contemplate the assessment and use of the information on the variables: total water content and salt content of the emulsion, aqueous phase pH, temperature, agitation, besides information related to the oil phase features such as viscosity, density, total acidity number (measured as TAN), asphaltene content, resin content, amount and kind of solids, and so on, in a method for treating water-in-oil emulsions and monitoring said treatment. Such variables determine the efficiency of the breaking of a water/highly viscous oil in the presence of microwave energy applied in a microwave apparatus in a method for treating water-in-oil emulsions and monitoring said treatment, said method being described and claimed in the present application.